Frostbite engine is such an unwieldy monster that it has killed the last 3 games that tried to use it, all of them top-tier AAA franchises no less (BFV, Anthem, ME:A).

It is optimized for two things, Battlefield and FIFA, and it is even cumbersome for that (see: BFV's lifecycle and the numerous long-standing bugs that plagued it). It was never designed for an RPG and things like inventory systems and facial animation did not exist and had to be invented (badly). But EA is all-in on the "everything has to run frostbite company-wide".

This characterization is inaccurate and misleading. A game engine is a basic framework with which to build a game - responsible for things like platform abstraction, memory management, and the basic rendering pipeline - but much of what is identifiably part of any specific game lives on top of that. Unreal Engine and Unity don’t ship with “inventory systems” and neither are “designed for an RPG”.

Having worked on multiple Frostbite and multiple Unreal games, both engines are capable of building a wide variety of games. The discrepancies in my experience aren’t technical but organizational. It is hard to compete with the scale of Epic’s developer support organization and the wider industry inertia around their technology.